The endemic dimorphic fungi are increasingly significant pathogens in clinical practice and cause substantial mortality in those with impaired immunity. These agents must convert from mold to yeast to acquire virulence and escape host defenses. Despite the pivotal role of morphogenesis in pathogenesis of these infections, the mechanisms that regulate the phase transition are poorly understood. The goal of my research is to uncover and characterize genes that regulate phase transition in dimorphic fungi using Blastomyces dermatitidis as a model. I hypothesize that genes involved in environmental sensing, intracellular signaling, cytokinesis, and morphology govern phase transition. To investigate this, I propose two aims with complementary approaches, using forward and reverse genetic strategies: 1) Elucidate the identity and function of genes in B. dermatitidis mutants defective in phase transition that I have already generated by Agrobacterium-mediated DNA transfer, and 2) Investigate the role in morphogenesis of two orthologs in the B. dermatitidis genome postulated to be involved in fungal phase transition - septin and phosphodiesterase - by knocking down their expression with RNA interference. My experimental approach will involve the use of cutting-edge molecular biology tools to discover new genes, discern their role in morphogenesis, and test their impact on virulence in a murine model of pulmonary infection. The results of my research will provide fundamental new insight about the molecular mechanisms behind phase transition in dimorphic fungi. The research training proposed will allow me to hone the skills I will need as a bench scientist to advance the field of molecular medical mycology. The coursework will give me a solid intellectual foundation in molecular genetics, microbial pathogenesis and medical mycology, while fulfilling the requirements for a Masters Degree. A mentoring committee with expertise in fungal pathogenesis will guide my progress in the lab and classroom. The overall training program will allow me to establish a contemporary research program investigating molecular regulation of morphogenesis in dimorphic fungi. The research proposed is relevant to public health because it will provide insight into the mechanisms by which fungi cause disease and may lead to the identification of new antifungal drug targets.